The organism that causes Johne’s disease is called Mycobacterium avium subspecies paratuberculosis (Mptb, or MAP). It is in the same family of bacteria as the ones that cause tuberculosis and leprosy in people. Mptb causes disease in ruminants – cattle, sheep, goats, deer, alpaca, camels and other cloven hoof animals, including wildlife.
When a ruminant ingests the bacterium it can lead to infection of the gut, particularly in the lower part of the small intestine. The disease usually takes a few years to develop in susceptible animals and can eventually cause weight loss and death.
Infected animals can look healthy for many years and it is these animals that ‘silently’ carry disease. This is most problematic. These animals shed the bug in their faeces, intermittently at first but later it is continuous, and in this way they spread the disease to the rest of the herd or flock, and to other herds and flocks.
The best test for diagnosing Johne’s disease requires culture of intestinal tissues. Unfortunately this is not a practical test. There are two main types of tests for detecting Johne’s disease in live animals: tests that detect the bug and tests that detect the immune response of the animal to the bug.
Tests that detect the bug mainly use faecal samples. Culture methods detect viable bugs but these tests take 8-12 weeks as Mptb grows very slowly. The new HT-J test developed by our group detects the DNA of the bug. This test takes only a few days, and is cheaper than culture.
Blood samples can be used to detect an animal’s immune response to the bug. The most common test is the antibody ELISA. In most animals antibodies take years to develop and are detected reliably mostly in animals in the later stages of disease. In many cases these animals are already infectious, shedding the bug in their faeces. So by the time an animal is positive in a blood test it has probably already passed the infection on to other animals.
Prevention is possible but requires a high level of biosecurity. This means maintaining good fencing to exclude stray animals, preferably with buffer zones if the disease is present on neighbouring farms. Stock should be purchased only from farms that participate in a Market Assurance Program for Johne’s disease, where there is regular herd/flock testing and certification of JD status. It is potentially risky to purchase stock from farms of unknown status within JD endemic regions.
On-farm management plans are best developed in consultation with an experienced veterinarian. In general, they involve preventing contact between young animals and adults that may shed the bug, preferably from birth. Grazing management, including spelling pastures, maintaining low stocking rates and rotational grazing of different species and age classes can be beneficial. Testing the herd/flock to identify and cull animals responsible for shedding the highest numbers of bacteria can also be undertaken.
Vaccination is another strategy used in the control of Johne’s disease. In Australia, the Gudair vaccine (Zoetis, formerly Pfizer) is registered for use in sheep. This vaccine can reduce mortality due to Johne’s disease and shedding of the bug by up to 90%. However some animals still become infected and shed high numbers of bacteria despite vaccination. Therefore it is recommended that all sheep are vaccinated.
The vaccine Silirum (Zoetis, formerly Pfizer) has recently been registered for use in cattle.